Wireless communications technologies are being widely applied in the field of public safety. LTE has been selected as a communications technology in a public safety network in some other countries, and spectrum selection for public safety networks is also under discussion in Europe, where LTE is an important candidate communications technology in the spectrum selection for public safety networks. In the field of public safety, occurrence of various disastrous situations such as fire and earthquakes needs to be considered. When these disasters occur, a base station may lose a connection to a core network and becomes an isolated base station, or a communication capacity between a base station and a core network is greatly limited and only communication between a part of control plane signaling and a core network may be ensured. In these cases, the base station is still expected to provide a service for a user equipment (UE) within coverage of the base station.
In an LTE communications system, a terminal is connected to a mobility management entity (MME)/service gateway (SGW) of a core network through an evolved base station (eNodeB), and is connected to a packet data network gateway (PGW, PDN) through the SGW. The terminal performs non-access stratum signaling interaction with the MME, and these pieces of signaling are transparently transmitted through the eNB.
Short distance discovery and communications technologies are applicable to public safety scenarios; however, because a distance between UEs is strictly limited in the short distance discovery and communications technologies, two UEs between which a distance is relatively far, for example, about 10 meters, cannot discover each other, let alone perform communication.